The types of proteins and the efficiency with which they are made is often the essential determinant of phenotypic expression in cells and organisms. Changes in protein synthetic ability can be modulated either by altering the messenger RNA content or by changes in the efficiency of their translation. Since the overall objective of this research program is to understand the mechanism of mammalian cell-growth regulation, we sought to examine certain aspects of mRNA structure and function in quiescent and serum induced proliferating Balb 3T3 clone A31 cells. The goals set for the current year were: to determine the mechanism of translational regulation in resting and serum induced 3T3 cells; secondly, to further characterize the protein associated with poly(A) regions in mRNA on the basis of its antigenic similarities; and thirdly, to focus on the structural features of a well characterized mRNA molecule in order to "map" the protein binding sites.